The present invention relates generally to atmospheric compensation and aim point maintenance for high-energy laser (HEL) weapon systems operated in the atmosphere, and in particular relates to the use of the high-energy laser itself for measuring atmospheric turbulence and for maintaining the HEL aim point.
In a high-energy laser weapon system, the laser needs to be maintained on a specific area of the target for a period of time to be effective. Atmospheric compensation using adaptive optics significantly reduces the time the high-energy laser must be maintained on the target. FIG. 1 illustrates the traditional HEL engagement geometry. A tracker illuminator (TIL) is employed to measure the angle and range of the target relative to the ground based or airborne HEL weapon system. A beacon illuminator (BIL) is used to create a pseudo star on the target. The BIL return signal is measured by a wavefront sensor to determine atmospheric turbulence between the weapon system and the target. This information is then used to drive adaptive optics (deformable mirrors) to vary the HEL beam to compensate for atmospheric disturbances. This reduces the time on target required for the HEL beam to destroy the target. For size, weight, and complexity limitations, the HEL and BIL systems are usually shared aperture designs that both transmit the high-energy laser and sense the target through the same telescope since building a second separate telescope and referencing the two to each other tends to be prohibitive. The high-energy laser is often pointed open loop with no feedback as to where it is on the target. The HEL can also be pointed with respect to the BIL, but with no direct feedback if it is hitting the aim point. However, direct feedback can be obtained by looking at the HEL scatter (in band) or heated spot (black body heating or hot spot). The beacon illuminator laser operates at a different frequency than the HEL and could need to be hundreds of watts of power to get enough return signal above the background and electronic noise depending on the range to the target.
FIG. 2 is a diagram of a traditional HEL weapon system optics illustrating the BIL system and how the HEL and BIL systems are integrated. The fast steering mirror 1 removes the jitter (tilt) by using the image generated by the TIL illumination of the target on the TIL track sensor 2. This is similar to what an image stabilizer does on a camera lens to remove jitter caused by the unsteadiness of the hand-held camera. The deformable mirror 3 “cleans-up” the image distorted by the atmosphere by tilting small portions of the image so that all the photons are going in the same direction. The atmosphere will make the image or outgoing HEL distort like a bathroom window with glass that doesn't let you see clearly. The deformable mirror 3 is driven by the output of the BIL wavefront sensor 4, which measures the distortion in the wavefront caused by the atmosphere. The deformable mirror 3 applies the opposite distortion so that when the HEL arrives at the target, the photons are all going in the same direction.
The aperture sharing element 5 is a beam splitter that samples a small part of the HEL and transmits the BIL. The BIL tilt sensor 6 measures direction of the outgoing BIL to drive the BIL steering mirror 7 to point it on the target at the lead ahead point and remove any jitter in the beam. The BIL beam polarizer 8 passes the outgoing BIL polarized beam and separates the two polarizations from the return beam for the BIL wavefront sensor 4. This loss of half the BIL return power is one of the reasons the BIL is required to have significant power in the outgoing beam. The other reason is the fact that the BIL is an unresolved point on the target and is subject to the range squared loss. If the range to the target is doubled, the return power to the sensor is one fourth. This is why it is desirable to use the HEL with all its power and the scatter off the target to drive the wavefront correction.
A beacon illuminator increases the complexity, power draw, and weight of the overall weapon system. One would like to use the high-energy laser itself as the beacon illuminator since it has much more power and is already pointed at the target. However, the outgoing beam of the high-energy laser is generally continuous and the scattered light from the optics is greater than the high-energy laser return from the target.
Examples of these limitations are the Airborne Laser with its kilowatt class beacon illuminators, complex optical system, high weight and large volume. It also does not sense the return continuous wave of the high-energy laser and therefore points the high-energy laser open loop (without any feedback from target about where it is being hit). Another example is the Advanced Tactical Laser that has no beacon illuminator due to the size and weight limitations of the small aircraft platform upon which it is mounted. Consequently, it cannot compensate for atmospheric turbulence resulting in greatly increased time on target to be effective.